A hydraulic system in an aircraft, for example, may include actuators or other powered devices which operate to cause movement of aerofoil surfaces such as ailerons or flaps, or for other purposes in the aircraft. These systems may operate most efficiently if the hydraulic fluid which operates the actuators or other power devices is at or above a preferred minimum temperature.
In a typical hydraulic system, hydraulic fluid is withdrawn from a reservoir, pressurized for operating hydraulic devices and the returned to the reservoir for recycling. As used herein, the pressurized hydraulic fluid is referred to as “working fluid” or the “high pressure fluid” which performs work in operating the hydraulic devices. It will be appreciated that returning hydraulic fluid which has been used to power the operation of an upstream hydraulically powered device, will be of a lower pressure than the working fluid which is fed to the hydraulically powered device. The returning fluid may also be at a lower temperature than a minimum temperature at which it is desired the fluid is maintained, for example less than 20° C.